The present invention relates generally to a process for the purification of aqueous solutions of tertiary amine-oxides.
The viscose process is presently widely used to produce cellulose molded bodies. For some decades, scientists have searched for a replacement process for the production of cellulose molded bodies. An interesting replacement process having reduced environmental impact has been discovered. In this method, cellulose is dissolved without derivatization in an organic solvent, and this solution is extruded to form molded bodies, e.g. fibers and films. Such extruded fibers have been given the generic name Lyocell by BISFA (The International Bureau for the Standardization of man made fibers). BISFA defines a mixture of an organic chemical substance and water as an organic solvent.
It has been shown that a mixture of a tertiary amine-oxide and water is particularly useful as an organic solvent for the production of cellulose molded bodies. N-methylmorpholine-N-oxide (NMMO) is typically used as the amine-oxide. Other suitable amine-oxides are described e.g. in EP-A 0 533 070. A method for the production of moldable cellulose solutions is known e.g. from EP-A-0 356 419.
In this process, cellulose is precipitated from a molded cellulose solution in an aqueous precipitation bath. During this process, amine-oxide builds up in the precipitation bath. To render this method economical it is of decisive importance to recover and reuse nearly all of the amine-oxide. Thus the amine-oxide process has the following 3 main steps:
(A) dissolving cellulose in an aqueous solution of a tertiary amine-oxide, preferably N-methylmorpholine-N-oxide (NMMO), to produce a moldable cellulose solution,
(B) molding said cellulose solution and conducting said molded cellulose solution into an aqueous precipitation bath wherein the cellulose is precipitated, thus producing a molded body and a spent precipitation bath,
(C) regenerating (i.e. purifying and concentrating) the spent precipitation bath, thus producing a regenerated aqueous amine-oxide solution which is reused in step (A) as a cellulose solvent.
During this process, amine-oxide and degradation products of cellulose and the amine-oxide build up in the precipitation bath. These products may be heavily colored. If the colored products are not removed from the precipitation bath, the quality of the produced molded bodies will be impaired. Metal traces may also build up in the precipitation bath, which reduces process safety.
There are some proposals in the literature for removing these degradation products before using the amine-oxide solution again in step (A). Specifically, DD-A 254 199 describes a known process for purifying aqueous solutions of NMMO, wherein the solution passes through anion exchangers. In the first step of this process, the anion exchanger contains an exchange resin comprising a styrene-divinylbenzene copolymerizate carrying tertiary amine-groups of the --CH.sub.2 N(CH.sub.3).sub.2 type. In the second step of this process, quaternary ammonium groups of the --CH.sub.2 N(CH.sub.3).sub.3 OH type act as functional groups. According to DD-A 254 199, NMMO solutions treated according to the process are dark at the beginning of the purification, brown to yellow after the first process step and bright yellow to transparent after the second process step.
A disadvantage of this process is that the treated solutions have a high pH value, which subsequently requires more complex purification. Additionally, this process does not remove alkali and alkali-earth cations from the solution. The metal ions, alkali metal ions and alkali-earth metal ions lead to undesired precipitations and incrustation, unwanted insoluble substances in the solution, and reduced process safety. Although it is possible to remove these salts by adding a precipitation agent and subsequently filtering or through the use of other separating agents, these operations are disadvantageous because they introduce additional chemicals or require additional processing.
EP-A-0 427 701 describes a second known process for purifying aqueous amine-oxide solutions. Specifically, EP-A-0 427 701 describes a process wherein the purification is carried out in a one-step process with an anion exchanger which exclusively has quaternary tetraalkyl ammonium functional groups according to the formulas --CH.sub.2 N.sup.+ (CH.sub.3).sub.3 X.sup.- or --CH.sub.2 N.sup.+ (CH.sub.3).sub.2 (CH.sub.2 OH)X.sup.-, wherein X.sup.- represents the anion of an inorganic or organic acid. In this process, the anion exchanger is regenerated using an aqueous acidic solution. The anion X.sup.- is preferably derived from a volatile acid, in particular carbonic acid, formic acid or acetic acid. These acids are also disclosed to be useful for regenerating the anion exchanger.
International Patent Application W093/11287 discloses regenerating the strong basic anion exchanger styrene-divinylbenzene copolymefizate by first using an aqueous solution of a strong inorganic acid and then soda lye (i.e. in two steps). The regeneration has to be carried out in two steps because the anion exchanger is colored so heavily by the solution to be purified that merely regenerating with aqueous NaOH does not suffice to remove the resin's color and regenerate its capacity. It is therefore only possible to achieve these results by treating the anion exchange a second time with a strong inorganic acid. This two-step procedure uses more chemicals and requires using strongly irritative substances (e.g. hydrochloric acid). Additionally it can be deduced from Example 5 of this document that the capacity of the anion exchanger is reduced to half of its original value after 10 operation cycles even when this two-step regeneration process is employed.
This styrene-divinylbenzene copolymerizate which is widely used as carrier in ion exchanging systems, is also disadvantageous because it will not rot and is difficult to reuse. Therefore, this ion exchanging system must after a certain time interval be incinerated as hazardous waste.
Alternative carriers are described in general technical manuals (see e.g. "Encyclopedia of Industrial Chemistry" by Ullmann, Volume A 14, page 396). Examples of such materials include polyacryl materials, phenol-formaldehyde resins or polyalkylamine resins. These materials must also eventually be incinerated as hazardous waste and usually must be produced using toxic or polluting starting materials (e.g. phenol-formaldehyde resins).